1. Field of the Invention
The present invention relates generally to optical navigation sensors, and more particularly to optical pointing devices used in computer systems.
2. Description of the Background Art
A computer mouse is an example of a two-dimensional navigation sensor. As employed with a personal computer, a mouse has an associated cursor displayed on-screen; the cursor moves relative to the mouse's movement. Some mice employ optical, rather than mechanical, technology to track movement. A conventional optical mouse includes a light source that illuminates an optically rough tracking surface at an oblique incident angle. The illuminated portion of the tracking surface is imaged to a tracking sensor, such as a CCD, CMOS imaging array, 2D comb array, etc. A tracking algorithm implemented in the mouse controller analyzes successive captured images or signals to determine mouse displacement along the tracking surface.
Conventional optical mice are designed to track well within a small margin around its nominal design height. Lift detection is implemented to prevent the mouse from tracking when lifted past a certain height threshold. This allows the user to lift and reposition the mouse without moving the cursor. The height threshold may be incorporated in the optical design of the mouse and may be based on image defocus, lateral field of view shift, or both. This works well for tracking surfaces that are directly under the mouse. However, the relatively small lift margin may prevent the mouse from tracking on an optically transparent tracking surface, such as a sheet of glass. This presents a problem in many office and residential applications where desks with sheets of glass are commonplace.